Trisomy 21 drives ADARB1 overexpression and premature RNA recoding in the developing fetal brain

We performed transcriptome-wide RNA sequencing on fetal cortical and hippocampal tissues from 20 T21 cases and 27 euploid controls collected between 13–22weeks post-conception—a critical period for human brain development. Differential expression analysis revealed 572 dysregulated genes in the cortex and 519 in the hippocampus (FDR < 5%), with significant enrichment for chromosome 21 genes.

Functional enrichment analyses highlighted disruptions in neurodevelopmental, synaptic, and immune-related pathways. Among the most strongly dysregulated genes was ADARB1, a chromosome 21–encoded RNA editing enzyme, whose overexpression in T21 fetal brain was associated with increased adenosine-to-inosine (A-to-I) editing, including key recoding events in GRIA2 (p.R764G), GRIA3 (p.R775G), and GRIK2 (p.Y571C, p.Q621R). A meta-analysis incorporating nine independent transcriptomic datasets spanning early embryonic and progenitor cell types validated robust chromosome 21 dosage effects, including consistent ADARB1 overexpression.